An Earth-like planet has been discovered around the nearest star to our own Sun but astronomers have discounted the possibility of it harbouring life because its molten surface stews at about 1,500C.

The planet is the closest and smallest among about 800 “exoplanets” that scientists have identified beyond the Solar System since 1995 when they detected the first extra-solar planet.

Little is known about the newly discovered planet except that it has a mass similar to Earth and spins around the our nearest star system, Alpha Centauri, once every 3.2 days at a distance of 6 million kilometres (3.7m miles).

Alpha Centauri is the brightest star in the southern sky and is just 4.3 light years from Earth – it would still take 40,000 years to travel there with existing rocket technology. It is actually composed of three stars, Alpha Centauri A and B and a third, more distant star known as Proxima Centauri.

The relatively close proximity of the new planet to Alpha Centauri B means that the planet is closer to its own star than the distance between Mercury – the innermost planet in the Solar System – and the Sun. This means the planet is too hot for either liquid water or life, or at least life as we know it.

However, the scientists believe that their success with detecting such a small object so far away means that they will soon be able to detect a twin planet to Earth in the “habitable zone” of its star, where liquid water and life can exist.

“The possibility of detecting an Earth-like planet in the habitable zone of this star system is good,” said Stephane [acute accent over first e] Urdy of the University of Geneva Observatory, a member of the study published in the journal Nature.

“This is the first planet with a mass similar to Earth ever found around a star like the Sun. Its orbit is very close to its star and it must be much too hot for life as we know it but it may well be just one planet in a system of several,” Dr Urdy said.

“It’s a landmark discovery because of the planet’s very low mass and because it’s our closet neighbouring star. It opens a good prospect of finding planets in the habitable zone of our closest star system,” he said.

Astronomers discovered the planet using a technique that monitors subtle changes to the wavelength of light coming from a star. This “Doppler wobble” of the star can indicate whether its movements are affected by the gravitational pull of a nearby planet as it completes each orbit.

It was the same technique – and same team of astronomers – who discovered the first exoplanet in 1995, a giant gas planet similar to Jupiter orbiting the star 51 Peg. However, the “Doppler wobble” technique has since been refined to enable the astronomers to detect Earth-sized planets about 150 times smaller than the giant planet found around 51 Peg.

The Alpha Centauri B planet was found using the HARPS instrument attached to the European Southern Observatory’s telescope at La Silla in Chile, which measures tiny changes in the speed at which the star is moving towards or away from the Earth.

The instrument effectively monitors the gravitational pull on the star caused by the planet and can do so down to a velocity of about 1.8km per hour, which is about the same speed of a crawling baby.

“Our observations extended over more than four years using the HARPS instrument and have revealed a tiny but real signal from a planet orbiting Alpha Centauri B every 3.2 days,” said Xavier Dumusque, also of the Geneva Observatory.

“It’s an extraordinary discovery and has pushed our technique to the limit,” Dr Dumusque said.